Heretofore, as a load measuring apparatus for detecting a load carried by a vehicle such as a truck, an apparatus equipped with a strain gauge or a magnetostriction sensor in a suspension system which deforms or warps in accordance with changes in the weight of load has been used. In the prior art load measuring apparatus, an output value of the strain gauge or magnetostriction sensor is detected, and the output value is converted into a display value indicative of the weight of load (e.g. Japanese Utility Model Laid-open Publication 06-69759).
In the prior art load measuring apparatus, when the temperature changes, the detection output of the strain gauge or magnetostriction sensor is offset depending on the characteristics of the strain gauge or magnetostriction sensor, and the weight of load was converted from the offset output value. Therefore, the display value of load weight was varied due to the change in temperature. In particular, when the vehicle is running under a high-load, low-speed condition, such as running on an ascending slope at full load, the suspension system tends to increase in temperature, resulting in substantial changes in the display value of the load weight.
Providing a separate temperature compensation sensor, which detects a temperature of an environment in which the load measuring sensor is disposed, attached to a structure of the vehicle where it is susceptible to the deformation and strain has also been considered. A difference between the output of the load measuring sensor and the output of the temperature compensation sensor was used to eliminate the influence of the temperature changes. However, the suspension system has no sufficient space for mounting the temperature compensation sensor, and use of such a sensor increases the number of sensors, making the apparatus more expensive.
In view of the above prior art problems, a primary object of the present invention is to provide a load measuring apparatus which does not change in load weight measurement output even when the temperature changes.